Distributed sensors based on Brillouin scattering are attractive for forming optical fiber sensing systems used to measure the structural integrity of buildings, bridges, tunnels, dams and pipelines, as well as ships and airplanes. The most popular Brillouin optical fiber sensing system is Brillouin Optical Time Domain Reflectometry (BOTDR). This technique is very similar to Rayleigh-based OTDR, where spontaneous Brillouin light backscattered from an intense pulse is recorded as a function of time. The frequency distribution of the backscattered signal is measured for each time step to determine a strain or a temperature change at each location. Like a conventional OTDR, a BOTDR requires access to a single fiber end only, which is convenient for many applications. However, the spatial resolution of BOTDR is practically limited to 1 m.
Another optical fiber sensing system utilizes Brillouin Optical Time Domain Analysis (BOTDA). This technique takes advantage of the Stimulated Brillouin Scattering (SBS) based on a pump-probe technique wherein an intense pump pulse interacts locally during its propagation with a weak counter-propagated continuous-wave (CW) probe. The gain experienced by the probe at each location can be analyzed by recording the probe amplitude in the time domain. The frequency difference between the pump and the probe is scanned step-by-step, and the local amplification can be retrieved for a given pump-probe frequency difference. The local gain spectrum can then be reconstructed by analyzing the gain at a given location as a function of frequency. BOTDA requires access to both optical fiber ends since the pump pulse and CW probe must counter-propagate in the sensing fiber, which is a limitation in some situations. BOTDA is subject to the similar spatial resolution limitation as BOTDR, i.e., about 1 m, because of 1) the tradeoff between spatial resolution and sensing sensitivity, and 2) the significant broadening and lowering of the Brillouin gain spectrum as the pulse width is reduced to the values comparable with the acoustic relaxation time (˜10 ns). In addition, one problem with conventional BOTDR or BOTDA systems in the field is the sensitivity of Brillouin frequency shift to both strain and temperature. This effect leads to ambiguity in the measurement, as one does not know whether the observed Brillouin frequency shift is caused by the change of strain or temperature.
To improve the spatial resolution, the use of a Brillouin Dynamic Grating (BDG) formed in a polarization-maintaining single-mode optical fiber has been proposed. In this approach, an acoustic wave is generated in one polarization by a pump and is used to reflect an orthogonally polarized probe at a different optical frequency from the pump. An experiment result was reported wherein a distributed strain is measured with a 10 cm spatial resolution in a 105 m of polarization-maintaining single-mode optical fiber. However, polarization-maintaining single-mode optical fibers typically have higher attenuation and are more expensive than standard optical fibers. In addition, the sensing schemes required for a polarization-maintaining optical single-mode fiber system require polarization-maintaining components that add additional cost and complexity to the sensing systems. Furthermore, it is difficult to maintain a given polarization over a long fiber (i.e., a few km or longer), which limits the sensor system length.
There is therefore a need for low-cost optical fiber sensing systems with improved spatial resolution and simultaneous measurement temperature and strain.